Scintillation crystal including a co-doped rare earth silicate, a radiation detection apparatus including the scintillation crystal, and a process of forming the same

ABSTRACT

A scintillation crystal can include a rare earth silicate, an activator, and a Group 2 co-dopant. In an embodiment, the Group 2 co-dopant concentration may not exceed 200 ppm atomic in the crystal or 0.25 at % in the melt before the crystal is formed. The ratio of the Group 2 concentration/activator atomic concentration can be in a range of 0.4 to 2.5. In another embodiment, the scintillation crystal may have a decay time no greater than 40 ns, and in another embodiment, have the same or higher light output than another crystal having the same composition except without the Group 2 co-dopant. In a further embodiment, a boule can be grown to a diameter of at least 75 mm and have no spiral or very low spiral and no cracks. The scintillation crystal can be used in a radiation detection apparatus and be coupled to a photosensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 17/130,439, entitled “SCINTILLATION CRYSTALINCLUDING A CO-DOPED RARE EARTH SILICATE, A RADIATION DETECTIONAPPARATUS INCLUDING THE SCINTILLATION CRYSTAL, AND A PROCESS OF FORMINGTHE SAME,” by Samuel Blahuta et al., filed Dec. 22, 2020, which is acontinuation of and claims priority to U.S. patent application Ser. No.16/804,133, entitled “SCINTILLATION CRYSTAL INCLUDING A CO-DOPED RAREEARTH SILICATE, A RADIATION DETECTION APPARATUS INCLUDING THESCINTILLATION CRYSTAL, AND A PROCESS OF FORMING THE SAME,” by SamuelBlahuta et al., filed Feb. 28, 2020, now U.S. Pat. No. 10,901,099, whichis a continuation of and claims priority to U.S. patent application Ser.No. 16/297,097, entitled “SCINTILLATION CRYSTAL INCLUDING A CO-DOPEDRARE EARTH SILICATE, A RADIATION DETECTION APPARATUS INCLUDING THESCINTILLATION CRYSTAL, AND A PROCESS OF FORMING THE SAME,” by SamuelBlahuta et al., filed Mar. 8, 2019, now U.S. Pat. No. 10,613,236, whichis a continuation of and claims priority to U.S. patent application Ser.No. 15/046,703, entitled “SCINTILLATION CRYSTAL INCLUDING A CO-DOPEDRARE EARTH SILICATE, A RADIATION DETECTION APPARATUS INCLUDING THESCINTILLATION CRYSTAL, AND A PROCESS OF FORMING THE SAME,” by SamuelBlahuta et al., filed Feb. 18, 2016, now U.S. Pat. No. 10,274,616, whichclaims priority to French Patent Application No. 15/00373, entitled“SCINTILLATION CRYSTAL INCLUDING A CO-DOPED RARE EARTH SILICATE, ARADIATION DETECTION APPARATUS INCLUDING THE SCINTILLATION CRYSTAL, AND APROCESS OF FORMING THE SAME,” by Samuel Blahuta et al., filed Feb. 26,2015, all of which are assigned to the current assignee hereof andincorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure is directed to scintillation crystals includingrare earth silicates and radiation detection apparatuses including suchscintillation crystals, and processes of forming the scintillationcrystals.

BACKGROUND

Lutetium oxyorthosilicates are commonly used in medical imagingradiation detectors. In some applications, part of the lutetium may bereplaced by yttrium, and in other applications, yttrium is not used. Ascintillation crystal can include a lutetium oxyorthosilicate that canbe co-doped with Ce and Ca to achieve a desired performance, such asgood light output and low decay time. Forming such scintillationcrystals at commercial production levels is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and are not limited in theaccompanying figures.

FIG. 1 includes photographs of boules that include a rare earth silicateat different concentrations of Ca.

FIG. 2 includes an illustration of a cross-sectional view of crystalpulling apparatus illustrating a spiral formed during crystal growth.

FIG. 3 includes an illustration of a radiation detection apparatus inaccordance with an embodiment that can be used in medical imaging.

FIG. 4 includes data obtained for Ca co-doped scintillation crystals.

FIG. 5 includes data obtained for Mg co-doped scintillation crystals.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of embodiments of the invention.

DETAILED DESCRIPTION

The following description in combination with the figures is provided toassist in understanding the teachings disclosed herein. The followingdiscussion will focus on specific implementations and embodiments of theteachings. This focus is provided to assist in describing the teachingsand should not be interpreted as a limitation on the scope orapplicability of the teachings.

Atomic percentages of dopants within a rare earth silicate scintillationcrystal or its corresponding melt are expressed relative to the totalrare earth elemental composition within the crystal or melt. Forexample, a melt may be formed from a combination of Lu₂O₃, Y₂O₃, CeO₂,and CaO. Calcium will have a concentration in the melt that is expressedwith the one or both of the following equations:

${{Ca}{at}\%} = {\frac{{Ca}{atoms}}{\left( {{{Lu}{atoms}} + {Y{atoms}} + {{Ce}{atoms}}} \right)} \times 100\%{or}}$${{Ca}{atomic}{ppm}} = {\frac{{Ca}{atoms}}{\left( {{{Lu}{atoms}} + {Y{atoms}} + {{Ce}{atoms}}} \right)} \times 1 \times {10^{6}.}}$

Group numbers corresponding to columns within the Periodic Table of theelements use the “New Notation” convention as seen in the CRC Handbookof Chemistry and Physics, 81^(st) Edition (2000-2001).

The term “rare earth” or “rare earth element” is intended to mean Y, Sc,and the Lanthanoid elements (La to Lu) in the Periodic Table of theElements.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having,” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of features is notnecessarily limited only to those features but may include otherfeatures not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive-or and not to an exclusive-or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

The use of “a” or “an” is employed to describe elements and componentsdescribed herein. This is done merely for convenience and to give ageneral sense of the scope of the invention. This description should beread to include one or at least one and the singular also includes theplural, or vice versa, unless it is clear that it is meant otherwise.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples are illustrative only and not intended to be limiting. To theextent not described herein, many details regarding specific materialsand processing acts are conventional and may be found in textbooks andother sources within the scintillation and radiation detection arts.

A scintillation crystal can include a rare earth silicate that isco-doped with an activator and a Group 2 element. The co-doping canimprove decay time, light yield, energy resolution, proportionality,another suitable scintillation parameter, or any combination thereof. Inan embodiment, the concentration of the Group 2 element, the ratio ofthe Group 2 element to the activator, or both may be controlled toobtain good scintillation performance and allow for crystal growth at acommercial production rate.

An activator is a specific type of dopant that affects the wavelengthfor the peak emission of a scintillation crystal. In a melt used to formthe scintillation crystal, Ce can be an activator in rare earthsilicates and may be present at a concentration of 0.11 at %. Ca can beused to reduce decay time of a scintillation crystal. Scintillatorsgrown from a melt containing 0.11 at % of Ce and Ca concentrations in arange of 0.1 at % to 0.2 at % have good decay times; however, commercialproduction of such scintillation crystals has been problematic. Theinventors have discovered that as larger boules or as a higherpercentage of Ca remains in a bath, the scintillation crystal growth canbecome unstable. A higher Ca concentration causes the viscosity of themelt to increase, surface tension to reduce, and heat transfer todecrease, any of which can cause the crystal growth to become unstable.

FIG. 1 includes four boules grown with different concentrations of Caand a diameter of approximately 55 mm. In a melt used to form thescintillation crystal, the rare earth content of each of the boulesincluded approximately 10 at % Y, 0.11 at % Ce, and the remainder (over88 at %) Lu. As the Ca concentration goes from 0.015 wt % to 0.2 wt %,as measured relative to SiO₂ content in the starting material, thespiral becomes more apparent. The dashed lines illustrate the portion ofboule that includes the spiral. Although not apparent in thephotographs, the boules with 0.1 and 0.2 wt % Ca had microcracks, andtherefore, were unacceptable for product quality scintillation crystals.At 0.015 wt %, the scintillation crystals had unacceptably long decaytimes (that is, longer than 40 ns).

The inventors have discovered that good (short) decay times can beachieved with relatively low Group 2 element concentrations. In anembodiment, the ratio of the Group 2 element to the activator can becontrolled within a range and still obtain good scintillationproperties. Thus, scintillation crystals can be produced in commercialsized boules having good scintillation properties, where such bouleshave no spiral of very low spiral.

FIG. 2 includes an illustration of a cross-sectional view of a portionof a crystal growth apparatus 20 that includes a crucible 22, a melt 24,and a crystal 26 grown from the bath. The crystal 26 includes a neck 261and a main body 263 having a diameter 265. Ideally, no spiral exists.However, if growth conditions are unstable, a spiral can be formed, suchas illustrated in FIG. 2 . The spiral can be defined by a depth 267,which is the depth of the groove formed by the spiral, and a length 269between successive peaks of the spiral. The length 269 per rotation ofthe spiral can be in a range of 1 time to 10 times the depth 267. Asused herein, no spiral, very low spiral, small spiral, and strong spiralcan be defined by the depths as set forth in Table 1 below.

TABLE 1 Degree of Spiral No spiral Depth 267 ≤0.2 mm Very low spiral 0.2mm < Depth 267 ≤1.5 mm Small spiral 1.5 mm < Depth 267 ≤ 4 mm Strongspiral 4.0 mm < Depth 267

A scintillation crystal can include a rare earth silicate and include anactivator and a co-dopant that includes a Group 2 element. In anembodiment, the concentration of the Group 2 element can be limited toreduce the likelihood of forming a spiral, and in a further embodiment,the ratio of atomic concentrations for Group 2 element/activator can becontrolled so that acceptable scintillation properties are achieved.Commercially sized boules (>75 mm diameter) can be formed with no spiralor very low spiral and achieve a decay time no greater than 40 ns. Thedecay time for a scintillation event is the time taken for the amplitudeof a light emission pulse to decrease from a maximum value to aspecified value (usually 10% of the maximum value).

In an embodiment, the Group 2 element can include Ca, Mg, Sr, or anymixture thereof. The Group 2 element concentration may not exceed 200ppm atomic based on the total rare earth content in the crystal or maynot exceed 0.25 at % based on the total rare earth content in the meltbefore crystal growth begins. The Group 2 element atomic concentrationdivided by the activator atomic concentration may be in a range of 0.4to 2.0 in the crystal and 0.4 to 2.5 in the melt before crystal growthbegins. The particular values for maximum Group 2 concentration andratio of Group 2 to activator atomic concentrations may depend on theparticular Group 2 element.

Regarding Ca in the scintillation crystal, in an embodiment, Ca has aconcentration in the scintillation crystal of at least 8 atomic ppm, atleast 11 atomic ppm, at least 15 atomic ppm, or at least 20 atomic ppmbased on a total rare earth content in the scintillation crystal, and inanother embodiment, Ca has a concentration in the scintillation crystalno greater than 160 atomic ppm, no greater than 150 atomic ppm, nogreater than 120 atomic ppm, or no greater than 95 atomic ppm based on atotal rare earth content in the scintillation crystal. In a particularembodiment, Ca has a concentration in the scintillation crystal in arange of 8 atomic ppm to 160 atomic ppm, 11 atomic ppm to 150 atomicppm, 15 atomic ppm to 120 atomic ppm, or 20 atomic ppm to 95 atomic ppmbased on a total rare earth content in the scintillation crystal.

The ratio of Ca atomic concentration divided by the activator atomicconcentration can be controlled. In an embodiment, the Ca atomicconcentration divided by the activator atomic concentration in thescintillation crystal is at least 0.50, at least 0.6, at least 0.7, or0.75, and in another embodiment, the Ca atomic concentration divided bythe activator atomic concentration in the scintillation crystal is nogreater than 2.0, no greater than 1.9, no greater than 1.8, or nogreater than 1.7. In a particular embodiment, the Ca atomicconcentration divided by the activator atomic concentration in thescintillation crystal is in a range of 0.50 to 2.0, 0.6 to 1.9, 0.7 to1.8, or 0.75 to 1.7.

When the Group 2 element is Mg instead of Ca, the values may bedifferent. Regarding the scintillation crystal, in an embodiment, Mg hasa concentration in the scintillation crystal of at least 50 atomic ppm,at least 70 atomic ppm, or at least 90 atomic ppm based on a total rareearth content in the scintillation crystal, and in another embodiment,Mg has a concentration in the scintillation crystal no greater than0.030 at %, no greater than 0.026 at %, or no greater than 0.022 at %based on a total rare earth content in the scintillation crystal. In aparticular embodiment, Mg has a concentration in the scintillationcrystal in a range of 50 atomic ppm to 0.030 at %, 70 atomic ppm to0.026 at %, or 90 atomic ppm to 0.022 at % based on a total rare earthcontent in the crystal.

Regarding the ratio of Mg atomic concentration divided by the activatoratomic concentration, in an embodiment, the Mg atomic concentrationdivided by the activator atomic concentration in the scintillationcrystal is at least 0.35, at least 0.37, or at least 0.40, and inanother embodiment, the Mg atomic concentration divided by the activatoratomic concentration in the scintillation crystal is no greater than2.5, no greater than 2.4, or no greater than 2.3. In a particularembodiment, the Mg atomic concentration divided by the activator atomicconcentration in the scintillation crystal is in a range of 0.35 to 2.5,0.37 to 2.4, or 0.40 to 2.3.

In an embodiment, the activator has a concentration in the scintillationcrystal of at least 20 atomic ppm, at least 25 atomic ppm, at least 28atomic ppm, or at least 30 atomic ppm based on a total rare earthcontent in the scintillation crystal, and in another embodiment, theactivator has a concentration in the scintillation crystal no greaterthan 1200 atomic ppm, no greater than 150 atomic ppm, no greater than120 atomic ppm, or no greater than 95 atomic ppm based on a total rareearth content in the scintillation crystal. In a particular embodiment,the activator has a concentration in the scintillation crystal in arange of 20 atomic ppm to 1200 atomic ppm, 25 atomic ppm to 150 atomicppm, 28 atomic ppm to 120 atomic ppm, or 30 ppm at to 95 atomic ppmbased on a total rare earth content in the scintillation crystal.

Although more testing is to be performed, the maximum concentration forSr is expected to be lower than Ca and Mg. The ratio of the Sr atomicconcentration to the activator atomic concentration is expected to besimilar to the corresponding ratios regarding Ca and Mg.

The likelihood of forming a spiral can increase as the diameter of theboule increases. By lowering the maximum content of the Group 2 elementwithin the crystal, the boule may be grown to a diameter of at least 75mm, at least 85 mm, at least 95 mm, or even larger with no spiral orvery low spiral. Furthermore, more of the initial charge forming thecrystal can be taken up by the boule before a small spiral is formed.Thus, at least 45 wt %, at least 50 wt %, at least 55 wt % or even morecan be used in forming the boule before a spiral is formed.

The scintillation crystal formed can have good scintillation properties,such as decay time. In an embodiment, the scintillation crystal has adecay time no greater than 40 ns, no greater than 38 ns, or no greaterthan 36 ns.

The scintillation crystal can have a formula of Ln₂SiO₅:Ac,Me orLn₂Si₂O₇:Ac,Me, wherein Ln includes one or more rare earth elementsdifferent from the activator, Ac is the activator; and Me is the Group 2element. In an embodiment, Ln is Y, Gd, Lu, or any combination thereof.In particular embodiment, Ln is Lu (for example, Lu₂SiO₅:Ac, Me orLu₂Si₂O₇:Ac, Me) or Lu_((1-x))Y_(x), wherein 0.0<x<1.0 (for example,Lu_(1.8)Y_(0.2)SiO₅:Ac, Me). In an embodiment, the activator can includeCe, Pr, Tb, another suitable rare earth activator, or any combinationthereof.

The concepts described herein are well suited for growing scintillationcrystals formed by a Czochralski, Kyropoulos, Edge-defined Film-FedGrowth (EFG), or Stepanov growth technique. The starting materials caninclude the corresponding oxides. For LYSO:Ce, Me, the startingmaterials can include Lu₂O₃, Y₂O₃, SiO₂, CeO₂, and MeCO₃, where Me is aGroup 2 element. Alternatively, the starting material can be LYSO:Ce, Methat has previously been reacted and may be in monocrystalline orpolycrystalline form. In still another embodiment, the startingmaterials may include a combination of the metal oxides and somepreviously reacted material. The starting materials can be ground to theappropriate size and thoroughly mixed. Referring to FIG. 2 , the processcan include charging the crucible 22 with an initial mass of thestarting materials.

The process can further include melting the charge to form the melt 24.The temperature of the melt 24 can depend on the composition of thescintillation crystal being formed. The melt 24 can include a rare earthsilicate and at least two dopants. One of the dopants can be anactivator, and the other dopant can include a Group 2 element. The Group2 element may be at a concentration in the melt that does not exceed0.25 at % before a boule is grown. The atomic concentration of the Group2 element divided by the atomic concentration of the activator can be ina range of 0.4 to 2.5 before the boule is grown.

Regarding Ca in the melt before crystal growth starts, in an embodiment,Ca has a concentration in the melt of at least 0.009 at %, at least0.012 at %, or at least 0.015 at % based on a total rare earth contentof the melt, and in another embodiment, Ca has a concentration in themelt no greater than 0.024 at %, no greater than 0.022 at %, or nogreater than 0.020 at % based on a total rare earth content of the melt.In a particular embodiment, Ca has a concentration in the melt in arange of 0.009 at % to 0.024 at % or 0.012 at % to 0.022 at % based on atotal rare earth content of the melt.

Regarding the dopant ratio, in an embodiment, the Ca atomicconcentration divided by the activator atomic concentration in the meltis at least 0.50, at least 0.6, at least 0.7, or at least 0.75 and inanother embodiment, the Ca atomic concentration divided by the activatoratomic concentration in the melt is no greater than 2.5, no greater than2.3, no greater than 2.1, or no greater than 1.7. In a particularembodiment, the Ca atomic concentration divided by the activator atomicconcentration in the melt is in a range of 0.50 to 2.5, 0.6 to 2.3, 0.7to 2.1, or 0.75 to 1.7.

Regarding Mg in the melt before crystal growth starts, in an embodiment,Mg has a concentration in the melt of at least 0.011 at %, at least0.020 at %, or at least 0.030 at % based on a total rare earth contentof the melt, and in another embodiment, Mg has a concentration in themelt no greater than 0.30 at %, no greater than 0.26 at %, or no greaterthan 0.22 at % based on a total rare earth content of the melt. In aparticular embodiment, Mg has a concentration in the melt in a range of0.011 at % to 0.30 at % or 0.020 at % to 0.26 at %, or 0.030 at % to0.22 at % based on a total rare earth content of the melt.

Regarding the dopant ratio, in an embodiment, the Mg atomicconcentration divided by the activator atomic concentration in the meltis at least 0.50, at least 0.6, at least 0.7, or at least 0.75, and inanother embodiment, the Mg atomic concentration divided by the activatoratomic concentration in the melt is no greater than 2.5, no greater than2.4, no greater than 2.3, or no greater than 1.7. In a particularembodiment, the Mg atomic concentration divided by the activator atomicconcentration in the melt is in a range of 0.50 to 2.5, 0.6 to 2.4, 0.7to 2.3, or 0.75 to 1.7.

In an embodiment, the activator has a concentration in the melt of atleast 0.011 at %, at least 0.02 at %, or at least 0.03 at % based on atotal rare earth content of the melt, and in another embodiment, theactivator has a concentration in the melt no greater than 0.20 at %, nogreater than 0.16 at %, or no greater than 0.12 at % based on a totalrare earth content of the melt. In a particular embodiment, theactivator has a concentration in the melt in a range of 0.011 at % to0.20 at %, 0.02 at % to 0.16 at %, or 0.03 at % to 0.12 at % based on atotal rare earth content of the melt.

The maximum concentration for Sr in the melt is expected to be lowerthan Ca and Mg. The ratio of the Sr atomic concentration to theactivator atomic concentration is expected to be similar to thecorresponding ratios regarding Ca and Mg.

The melt can be at a temperature of at least 1700° C., at least 1800°C., at least 1900° C., or even higher. The crucible 22 can be made fromiridium, and thus, the melt 24 may be at a temperature no greater than2500° C., no greater than 2400° C., or no greater than 2300° C.

The process can include growing a boule from the melt 24. Unlike theboule 26 illustrated in FIG. 2 , the boule grown with a compositionpreviously described has no spiral or very low spiral. The main body ofthe boule (below the neck region) can have a diameter of at least 75 mm,85 mm, or 95 mm. The diameter can be up to 105 mm for a 150 mm diameterIr crucible. During the growth, at least 45 wt %, at least 50 wt %, atleast 66 wt %, or possibly more of the initial charge can be grown intothe boule. As more of the boule is grown, the concentrations of thedopants can increase within the melt. By keeping the concentration ofthe Group 2 element in the starting material relatively low, having aratio of the atomic concentration of the Group 2 element to the atomicconcentration of the activator in the starting material within apredetermined range, or both, the boule can be grown with no spiral orvery low spiral. After the boule is grown, it can be cut to obtainscintillation crystals. In a particular embodiment, the scintillationcrystals have a rare earth silicate composition and a decay time nogreater than 40 ns.

Any of the scintillation crystals as previously described can be used ina variety of applications. Exemplary applications include gamma rayspectroscopy, isotope identification, Single Positron Emission ComputerTomography (SPECT) or Positron Emission Tomography (PET) analysis, x-rayimaging, oil well-logging detectors, and detecting the presence ofradioactivity. The scintillation crystal is particularly well suited forapplications in which timing of scintillation events is relatively moreimportant, such as medical imaging applications. Time of flight PET anddepth of interaction PET apparatuses are specific examples of apparatusthat can be used for such medical imaging applications. Thescintillation crystal can be used for other applications, and thus, thelist is merely exemplary and not limiting. A couple of specificapplications are described below.

FIG. 3 illustrates an embodiment of a radiation detection apparatus 300that can be used for gamma ray analysis, such as a Single PositronEmission Computer Tomography (SPECT) or Positron Emission Tomography(PET) analysis. Applications using PET can include TOF PET Time ofFlight Positron Emission Tomography (TOP PET) imaging, Depth ofInteraction Positron Emission Tomography (DOI PET) imaging, or both.Hybrid applications involving PET imaging can include Positron EmissionTomography with Computed Tomography capabilities (PET/CT), PositronEmission Tomography with Magnetic Resonance capabilities (PET/MR) andPositron Emission Tomography with Single Photon Emission ComputedTomography capabilities (PET/SPECT).

In the embodiment illustrated, the radiation detection apparatus 300includes a photosensor 301, an optical interface 303, and ascintillation device 305. Although the photosensor 301, the opticalinterface 303, and the scintillation device 305 are illustrated separatefrom each other, skilled artisans will appreciate that photosensor 301and the scintillation device 305 can be coupled to the optical interface303, with the optical interface 303 disposed between the photosensor 301and the scintillation device 305. The scintillation device 305 and thephotosensor 301 can be optically coupled to the optical interface 303with other known coupling methods, such as the use of an optical gel orbonding agent, or directly through molecular adhesion of opticallycoupled elements.

The photosensor 301 may be a photomultiplier tube (PMT), asemiconductor-based photomultiplier (for example, SiPM), or a hybridphotosensor. The photosensor 301 can receive photons emitted by thescintillation device 305, via an input window 316, and produceelectrical pulses based on numbers of photons that it receives. Thephotosensor 301 is electrically coupled to an electronics module 330.The electrical pulses can be shaped, digitized, analyzed, or anycombination thereof by the electronics module 330 to provide a count ofthe photons received at the photosensor 301 or other information. Theelectronics module 330 can include an amplifier, a pre-amplifier, adiscriminator, an analog-to-digital signal converter, a photon counter,a pulse shape analyzer or discriminator, another electronic component,or any combination thereof. The photosensor 301 can be housed within atube or housing made of a material capable of protecting the photosensor301, the electronics module 330, or a combination thereof, such as ametal, metal alloy, other material, or any combination thereof.

The scintillation device 305 includes a scintillation crystal 307 can beany one of the scintillation crystals previously described. Thescintillation crystal 307 is substantially surrounded by a reflector309. In one embodiment, the reflector 309 can includepolytetrafluoroethylene (PTFE), another material adapted to reflectlight emitted by the scintillation crystal 307, or a combinationthereof. In an illustrative embodiment, the reflector 309 can besubstantially surrounded by a shock absorbing member 311. Thescintillation crystal 307, the reflector 309, and the shock absorbingmember 311 can be housed within a casing 313.

The scintillation device 305 includes at least one stabilizationmechanism adapted to reduce relative movement between the scintillationcrystal 307 and other elements of the radiation detection apparatus 300,such as the optical interface 303, the casing 313, the shock absorbingmember 311, the reflector 309, or any combination thereof. Thestabilization mechanism may include a spring 319, an elastomer, anothersuitable stabilization mechanism, or a combination thereof. Thestabilization mechanism can be adapted to apply lateral forces,horizontal forces, or a combination thereof, to the scintillationcrystal 307 to stabilize its position relative to one or more otherelements of the radiation detection apparatus 300.

As illustrated, the optical interface 303 is adapted to be coupledbetween the photosensor 301 and the scintillation device 305. Theoptical interface 303 is also adapted to facilitate optical couplingbetween the photosensor 301 and the scintillation device 305. Theoptical interface 303 can include a polymer, such as a silicone rubber,which is polarized to align the reflective indices of the scintillationcrystal 307 and the input window 316. In other embodiments, the opticalinterface 303 can include gels or colloids that include polymers andadditional elements.

In a further embodiment, an array of scintillation crystals can becoupled to photosensors to provide a high resolution image.

Embodiments as described herein can allow for good performingscintillation crystals to be formed at commercial production rates. Therelatively lower content of Group 2 elements can help to reduce thelikelihood that a spiral will form during boule growth, particularlywhen the boule diameter is 75 mm and larger, when at least 45 wt % ofthe initial charge of a melt is used in forming the boule, or both.Furthermore, strain within the boule will be relatively lower and lesslikely to form cracks. The ratio of the Group 2 and activator atomicconcentrations can be controlled and still achieve good light outputwith acceptable decay times. Thus, scintillation crystals can be formedat commercial production rates and still achieve good scintillationproperties.

Many different aspects and embodiments are possible. Some of thoseaspects and embodiments are described herein. After reading thisspecification, skilled artisans will appreciate that those aspects andembodiments are only illustrative and do not limit the scope of thepresent invention. Embodiments may be in accordance with any one or moreof the embodiments as listed below.

Embodiment 1. A scintillation crystal comprising a rare earth silicate,a first dopant, and a second dopant, wherein: the first dopant is anactivator and has a first concentration; the second dopant includes aGroup 2 element and has a second concentration that does not exceed 200ppm atomic based on a total rare earth content of the scintillationcrystal; and the second atomic concentration divided by the first atomicconcentration is in a range of 0.4 to 2.0.

Embodiment 2. A scintillation crystal comprising a rare earth silicate,a first dopant, and a second dopant, wherein: the first dopant is anactivator and has a first concentration; the second dopant includes aGroup 2 element and has a second concentration that does not exceed 200ppm atomic based on a total rare earth content of the scintillationcrystal; and the scintillation crystal has a decay time no greater than40 ns.

Embodiment 3. A process of forming a scintillation crystal comprising:charging a crucible with an initial mass of a material for thescintillation crystal that includes a rare earth silicate, a firstdopant, and a second dopant, wherein: the first dopant is an activatorand has a first concentration; and the second dopant is a Group 2element; melting the charge to form a melt that includes a rare earthsilicate, a first dopant, and a second dopant, wherein: the first dopanthas a first concentration in the melt; the second dopant has a secondconcentration in the melt that does not exceed 0.25 at % based on atotal rare earth content of the melt; and the second atomicconcentration divided by the first atomic concentration is in a range of0.4 to 2.5; growing a boule from the melt; and cutting the boule toobtain a scintillation crystal.

Embodiment 4. The process of Embodiment 3, wherein growing the boule isperformed such that the boule includes at least 45 wt % of the initialmass and has no spiral or very low spiral.

Embodiment 5. A process of forming a scintillation crystal comprising:charging a crucible with an initial mass of a material for thescintillation crystal that includes a rare earth silicate, a firstdopant, and a second dopant, wherein: the first dopant is an activator;and the second dopant is a Group 2 element; melting the charge to form amelt; growing a boule from the melt, wherein the boule includes at least45 wt % of the initial mass and has no spiral or very low spiral; andcutting the boule to obtain a scintillation crystal that has a decaytime no greater than 40 ns.

Embodiment 6. The process of any one of Embodiments 3 to 5, wherein theboule has a diameter of at least 75 mm, at least 85 mm, or at least 95mm.

Embodiment 7. The process of any one of Embodiments 3 to 6, whereingrowing the boule is performed using a Czochralski or Kyropoulos growthtechnique.

Embodiment 8. The process of any one of Embodiments 3 to 7, wherein atemperature of the melt is at least 1700° C., at least 1800° C., or atleast 1900° C.

Embodiment 9. The process of any one of Embodiments 3 to 8, wherein verylow spiral has: a depth of a spiral, as measured normal to a sidesurface of the boule, is no greater than 1.5 mm; and a step of thespiral is in a range of 1 to 10 times the depth.

Embodiment 10. The process of any one of Embodiments 3 to 9, wherein thesecond dopant is Ca and has a concentration in the melt of at least0.009 at %, at least 0.012 at %, or at least 0.015 at % based on a totalrare earth content of the melt.

Embodiment 11. The process of any one of Embodiments 3 to 10, whereinthe second dopant is Ca and has a concentration in the melt no greaterthan 0.024 at %, no greater than 0.022 at %, or no greater than 0.020 at% based on a total rare earth content of the melt.

Embodiment 12. The process of any one of Embodiments 3 to 11, whereinthe second dopant is Ca and has a concentration in the melt in a rangeof 0.009 at % to 0.024 at % or 0.012 at % to 0.022 at % based on a totalrare earth content of the melt.

Embodiment 13. The process of any one of Embodiments 3 to 12, whereinthe second dopant is Ca, and the second atomic concentration divided bythe first atomic concentration in the melt is at least 0.50, at least0.6, or at least 0.7.

Embodiment 14. The process of any one of Embodiments 3 to 13, whereinthe second dopant is Ca, and the second atomic concentration divided bythe first atomic concentration in the melt is no greater than 2.5, nogreater than 2.3, no greater than 2.1, or no greater than 1.7.

Embodiment 15. The process of any one of Embodiments 3 to 14, whereinthe second dopant is Ca, and the second atomic concentration divided bythe first atomic concentration in the melt is in a range of 0.50 to 2.5,0.6 to 2.3, 0.7 to 2.1, or 0.75 to 1.7.

Embodiment 16. The process of any one of Embodiments 3 to 9, wherein thesecond dopant is Mg and has a concentration in the melt of at least0.011 at %, at least 0.020 at %, or at least 0.030 at % based on a totalrare earth content of the melt.

Embodiment 17. The process of any one of Embodiments 3 to 9 and 16,wherein the second dopant is Mg and has a concentration in the melt nogreater than 0.30 at %, no greater than 0.26 at %, or no greater than0.22 at % based on a total rare earth content of the melt.

Embodiment 18. The process of any one of Embodiments 3 to 9, 16, and 17,wherein the second dopant is Mg and has a concentration in the melt in arange of 0.011 at % to 0.30 at % or 0.020 at % to 0.26 at %, or 0.030 at% to 0.22 at % based on a total rare earth content of the melt.

Embodiment 19. The process of any one of Embodiments 3 to 9 and 16 to18, wherein the second dopant is Mg, and the second atomic concentrationdivided by the first atomic concentration in the melt is at least 0.50,at least 0.6, at least 0.7, or at least 0.75.

Embodiment 20. The process of any one of Embodiments 3 to 9 and 16 to19, wherein the second dopant is Mg, and the second atomic concentrationdivided by the first atomic concentration in the melt is no greater than2.5, no greater than 2.4, no greater than 2.3, or no greater than 1.7.

Embodiment 21. The process of any one of Embodiments 3 to 9 and 16 to20, wherein the second dopant is Mg, and the second atomic concentrationdivided by the first atomic concentration in the melt is in a range of0.50 to 2.5, 0.6 to 2.4, 0.7 to 2.3, or 0.75 to 1.7.

Embodiment 22. The process of any one of Embodiments 3 to 21, whereinthe first dopant has a concentration in the melt of at least 0.011 at %,at least 0.02 at %, or at least 0.03 at % based on a total rare earthcontent of the melt.

Embodiment 23. The process of any one of Embodiments 3 to 22, whereinthe first dopant has a concentration in the melt no greater than 0.20 at%, no greater than 0.16 at %, or no greater than 0.12 at % based on atotal rare earth content of the melt.

Embodiment 24. The process of any one of Embodiments 3 to 23, whereinthe first dopant has a concentration in the melt in a range of 0.011 at% to 0.20 at %, 0.02 at % to 0.16 at %, or 0.03 at % to 0.12 at % basedon a total rare earth content of the melt.

Embodiment 25. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein the scintillation crystal has a decaytime no greater than 40 ns, no greater than 38 ns, or no greater than 36ns.

Embodiment 26. The scintillation crystal or the process of Embodiment25, wherein the scintillation crystal has a decay time of at least 15ns.

Embodiment 27. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein the scintillation crystal has aformula of Ln₂SiO₅:Ac,Me or Ln₂Si₂O₇:Ac,Me, wherein: Ln includes one ormore rare earth elements different from the first dopant; Ac is thefirst dopant; and Me is the second dopant.

Embodiment 28. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein Ln is Y, Gd, Lu, or any combinationthereof.

Embodiment 29. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein Ln is Lu or Lu_((1-x))Y_(x), wherein0.0<x<1.0.

Embodiment 30. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein first dopant is Ce, Pr, Tb, or anycombination thereof.

Embodiment 31. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 25 to 30, wherein the second dopant is Ca andhas a concentration in the scintillation crystal of at least 8 atomicppm, at least 11 atomic ppm, at least 15 atomic ppm, or at least 20atomic ppm based on a total rare earth content in the scintillationcrystal.

Embodiment 32. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 25 to 31, wherein the second dopant is Ca andhas a concentration in the scintillation crystal no greater than 160atomic ppm, no greater than 150 atomic ppm, no greater than 120 atomicppm, or no greater than 95 atomic ppm based on a total rare earthcontent in the scintillation crystal.

Embodiment 33. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 25 to 32, wherein the second dopant is Ca andhas a concentration in the scintillation crystal in a range of 8 atomicppm to 160 atomic ppm, 11 atomic ppm to 150 atomic ppm, 15 atomic ppm to120 atomic ppm, or 20 atomic ppm to 95 atomic ppm based on a total rareearth content in the scintillation crystal.

Embodiment 34. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 25 to 33, wherein the second dopant is Ca, andthe second atomic concentration divided by the first atomicconcentration in the scintillation crystal is at least 0.50, at least0.6, at least 0.7, or at least 0.75.

Embodiment 35. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 25 to 34, wherein the second dopant is Ca, andthe second atomic concentration divided by the first atomicconcentration in the scintillation crystal is no greater than 2.0, nogreater than 1.9, no greater than 1.8, or no greater than 1.7.

Embodiment 36. The scintillation crystal or the process of any one ofEmbodiments 1 to 15 and 28 to 35, wherein the second dopant is Ca, andthe second atomic concentration divided by the first atomicconcentration in the scintillation crystal is in a range of 0.50 to 2.0,0.6 to 1.9, 0.7 to 1.8, or 0.75 to 1.7.

Embodiment 37. The scintillation crystal or the process of any one ofEmbodiments 1 to 9 and 16 to 30, wherein the second dopant is Mg and hasa concentration in the scintillation crystal of at least 50 atomic ppm,at least 70 atomic ppm, or at least 90 atomic ppm based on a total rareearth content in the scintillation crystal.

Embodiment 38. The scintillation crystal or the process of any one ofEmbodiments 1 to 9, 16 to 30, and 37, wherein the second dopant is Mgand has a concentration in the scintillation crystal no greater than0.030 at %, no greater than 0.026 at %, or no greater than 0.022 at %based on a total rare earth content in the scintillation crystal.

Embodiment 39. The scintillation crystal or the process of any one ofEmbodiments 1 to 9, 16 to 30, 37, and 38, wherein the second dopant isMg and has a concentration in the scintillation crystal in a range of 50atomic ppm to 0.030 at %, 70 atomic ppm to 0.026 at %, or 90 atomic ppmto 0.022 at % based on a total rare earth content in the crystal.

Embodiment 40. The scintillation crystal or the process of any one ofEmbodiments 1 to 9, 16 to 30, and 37 to 39, wherein the second dopant isMg, and the second atomic concentration divided by the first atomicconcentration in the scintillation crystal is at least 0.35, at least0.37, or at least 0.40.

Embodiment 41. The scintillation crystal or the process of any one ofEmbodiments 1 to 9, 19 to 33, and 37 to 40, wherein the second dopant isMg, and the second atomic concentration divided by the first atomicconcentration in the scintillation crystal is no greater than 2.5, nogreater than 2.4, or no greater than 2.3.

Embodiment 42. The scintillation crystal or the process of any one ofEmbodiments 1 to 9, 16 to 30, and 37 to 41, wherein the second dopant isMg, and the second atomic concentration divided by the first atomicconcentration in the scintillation crystal is in a range of 0.35 to 2.5,0.37 to 2.4, or 0.40 to 2.3.

Embodiment 43. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein the first dopant has a concentrationin the scintillation crystal of at least 20 atomic ppm, at least 25atomic ppm, at least 28 atomic ppm, or at least 30 atomic ppm based on atotal rare earth content in the scintillation crystal.

Embodiment 44. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein the first dopant has a concentrationin the scintillation crystal no greater than 1200 atomic ppm, no greaterthan 150 atomic ppm, no greater than 120 atomic ppm, or no greater than95 atomic ppm based on a total rare earth content in the scintillationcrystal.

Embodiment 45. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein the first dopant has a concentrationin the scintillation crystal in a range of 20 atomic ppm to 1200 atomicppm, 25 atomic ppm to 150 atomic ppm, 28 atomic ppm to 120 atomic ppm,or 30 atomic ppm to 95 atomic ppm based on a total rare earth content inthe scintillation crystal.

Embodiment 46. The scintillation crystal or the process of any one ofthe preceding Embodiments, wherein a light output of the scintillationcrystal is greater a light output of another scintillation crystalhaving the same composition except without the second dopant.

Embodiment 47. A radiation detection apparatus comprising: thescintillation crystal of any one of Embodiments 1, 2, and 25 to 46 ormade by the process of any one of Embodiments 3 to 46; and a photosensoroptically coupled to the scintillation crystal.

Embodiment 48. The radiation detection apparatus of Embodiment 47,wherein the radiation detection apparatus is an imaging apparatus.

Embodiment 49. The radiation detection apparatus of Embodiment 47 or 48,wherein the radiation detection apparatus is a positron emissiontomography or a single photon emission computed tomography apparatus.

EXAMPLES

The concepts described herein will be further described in the Examples,which do not limit the scope of the invention described in the claims.The Examples demonstrate performance of scintillation crystals ofdifferent compositions. Numerical values as disclosed in this Examplessection may be averaged from a plurality of readings, approximated, orrounded off for convenience. Samples were formed using a Czochralskicrystal growing technique. The scintillation crystals were principallyLu_(1.8)Y_(0.2)SiO₅ with the activator and the co-dopant being the onlyintentionally added impurities. The examples demonstrate that boules of100 mm diameter can be formed with no spiral or very low spiral and gooddecay time.

FIGS. 4 and 5 include tables with data for Ca and Mg co-dopedscintillation crystals. Diameters and concentrations of the Group 2element and Ce were varied. The boule shape and scintillation crystalswere analyzed for light output in absolute values calculated inphotons/MeV at the ¹³⁷Cs radiation source at 662 keV, such absolutevalues were divided by the light output of a referenced standard sampleof LYSO:Ce without co-doping and multiplied by 100%. Examples withoutany co-dopant were used as a reference. For each boule with twoexamples, the first example was closer to the top of the boule, and thesecond example was closer to the bottom of the boule.

As expected, the reference (Examples 1 and 2) that only included Ce as adopant (no Group 2 co-dopant), the boule exhibited no spiral and thedecay time was slow and in a range of 45-50 ns. Relatively low Caconcentrations (0.011 at %) when Ce is 0.11 at % in the melt (Examples 3to 5) exhibited no spiral. The relatively low Ca concentration combinedwith a Ce concentration used for the reference had a relatively lowCa/Ce ratio. Scintillation crystals formed from such boules had decaytimes in a range of 43 to 49 ns.

For Examples 6 and 7, not all of the data was available. At a Caconcentration of 0.022 at % in the melt, the boule exhibited very lowspiral. However, with the Ca/Ce atomic ratio being relatively low,scintillation crystals from such a boule may have decay times greaterthan 40 ns.

Examples 8 to 11 had relatively high Ca concentrations (0.15 at % in themelt). The boules exhibited strong spirals and the decay times were in arange of 32.7 to 36.5 ns. The boules have cracks and would beunacceptable as product.

Examples 12 to 21 had Ca concentrations of 0.018-0.022 at % in the meltand Ca/Ce ratios of 1.10 to 1.48 in the melt and formed boules with nospiral or very low spiral. Scintillation crystals formed such boules hadCa concentrations of 29-82 atomic ppm in the crystal and Ca/Ce ratios of0.56 to 1.74 in the crystal. Light output was good (102-129%), low decaytimes were 34.7-38.5 ns. Examples 12 and 13 had a nominal diameter of 50mm (2 inch) and exhibited very low spiral. If the composition forExamples 12 and 13 were scaled up to 100 mm, the degree of spiral may besmall or strong. Thus, the Ca concentration in Examples 12 and 13 may betoo high for large diameter boules; however further testing would beneeded to confirm.

Examples 22 and 23 have a relatively high Ca concentration (0.075 at %in the melt) as compared to many of the other examples. Similar to otherhigh Ca concentration samples, the boule exhibited a small spiral. Thus,the composition for Examples 22 and 23 would not be a good candidate forscaling the boule from the 50 mm nominal diameter to a 75 mm nominaldiameter or larger boule.

Examples 24 and 25 had compositions where the Ca concentration was 0.022at % in the melt and a Ca/Ce atomic ratio of 2.44. The boule exhibitedno spiral. Example 24 corresponds to the top of the boule and had a Caconcentration of 22 atomic ppm and a Ca/Ce atomic ratio of 0.79, whichis similar to Example 20. Example 25 corresponds to the top of the bouleand had a Ca concentration of 60 atomic ppm and a Ca/Ce atomic ratio of1.13, which is similar to Example 19. Although decay times were notmeasured for Examples 24 and 25, they are expected to be similar toExamples 20 and 19, respectively. Thus, the decay times for Examples 24and 25 are expected to be good and in a range of approximately 35.5-38.0ns, based on the data from Examples 19 and 20.

Examples 26 to 29 have Mg as the co-dopant. The boules for Examples 26to 29 were grown from a melt that included 0.011 at % Ce and 0.247 at %Mg, had a nominal diameter of 50 mm, and exhibited very low spiral.Examples 26 and 27 had an average Mg concentration of 163 atomic ppm,had an average Mg/Ce atomic ratio of 0.45, and a decay time of 38.9-39.6ns. Examples 28 and 29 had an average Mg concentration of 141 atomicppm, had an average Mg/Ce atomic ratio of 0.36, and a decay time of39.3-40.9 ns. Further testing of Mg as a co-dopant may allow for asmaller decay time and a lower likelihood of forming a spiral as theboule diameter is increased.

Note that not all of the activities described above in the generaldescription or the examples are required, that a portion of a specificactivity may not be required, and that one or more further activitiesmay be performed in addition to those described. Still further, theorder in which activities are listed is not necessarily the order inwhich they are performed.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any feature(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature of any or all the claims.

The specification and illustrations of the embodiments described hereinare intended to provide a general understanding of the structure of thevarious embodiments. The specification and illustrations are notintended to serve as an exhaustive and comprehensive description of allof the elements and features of apparatus and systems that use thestructures or methods described herein. Separate embodiments may also beprovided in combination in a single embodiment, and conversely, variousfeatures that are, for brevity, described in the context of a singleembodiment, may also be provided separately or in any subcombination.Further, reference to values stated in ranges includes each and everyvalue within that range. Many other embodiments may be apparent toskilled artisans only after reading this specification. Otherembodiments may be used and derived from the disclosure, such that astructural substitution, logical substitution, or another change may bemade without departing from the scope of the disclosure. Accordingly,the disclosure is to be regarded as illustrative rather thanrestrictive.

What is claimed is:
 1. A scintillation crystal comprising a rare earthsilicate, a first dopant, and a second dopant, wherein: the first dopantis an activator and has a first atomic concentration; the second dopantincludes a Group 2 element and has a second atomic concentration that isat least 8 ppm atomic based on a total rare earth content of thescintillation crystal.
 2. The scintillation crystal of claim 1, whereinthe scintillation crystal has a decay time of no greater than 38 ns. 3.The scintillation crystal of claim 1, wherein the scintillation crystalhas a decay time of at least 15 ns and no greater than 40 ns.
 4. Thescintillation crystal of claim 1, wherein the second atomicconcentration does not exceed 200 ppm atomic based on the total rareearth content of the scintillation crystal.
 5. The scintillation crystalof claim 1, wherein the second atomic concentration is at least 20atomic ppm based on the total rare earth content in the scintillationcrystal.
 6. The scintillation crystal of claim 1, wherein the firstdopant is Ce, Pr, Tb, or any combination thereof.
 7. The scintillationcrystal of claim 1, wherein the first atomic concentration in thescintillation crystal is at least 20 atomic ppm based on a total rareearth content in the scintillation crystal.
 8. The scintillation crystalof claim 1, wherein the Group 2 element is selected from the groupconsisting of Ca, Mg, and Sr.
 9. The scintillation crystal of claim 8,wherein the Group 2 element is Ca.
 10. The scintillation crystal ofclaim 8, wherein the Group 2 element has a second atomic concentrationbetween 8 ppm and 3000 ppm atomic based on a total rare earth content ofthe scintillation crystal.
 11. The scintillation crystal of claim 1,wherein the second atomic concentration in the scintillation crystal isno greater than 150 atomic ppm based on a total rare earth content inthe scintillation crystal.
 12. A scintillation crystal comprising aformula of Lu_((1-x))Y_(x)SiO₅:Ac, Me, wherein: Ac is a first dopant andhas a first atomic concentration in the scintillation crystal of atleast 20 atomic ppm based on a total rare earth content in thescintillation crystal, wherein the first dopant is Ce, Pr, Tb, or anycombination thereof; Me is a second dopant and selected from the groupconsisting of Ca, Mg, and Sr and has a second atomic concentration thatis at least 20 ppm atomic based on a total rare earth content of thescintillation crystal; wherein 0<x<1; and wherein the scintillationcrystal has a decay time of no greater than 40 ns.
 13. The scintillationcrystal of claim 12, wherein the second atomic concentration does notexceed 200 ppm atomic based on the total rare earth content of thescintillation crystal.
 14. The scintillation crystal of claim 12,wherein the second atomic concentration is at least 15 atomic ppm basedon a total rare earth content in the scintillation crystal.
 15. Ascintillation crystal comprising a formula of Lu_((1-x))Y_(x)SiO₅:Ce,Me, wherein: Ce has a first atomic concentration in the scintillationcrystal of at least 20 atomic ppm based on a total rare earth content inthe scintillation crystal; Me is a Group 2 element selected from thegroup consisting of Ca, Mg, and Sr, and wherein the Group 2 element hasa second atomic concentration between 8 ppm and 3000 ppm atomic based ona total rare earth content of the scintillation crystal; wherein 0<x<1;and wherein scintillation crystal has a decay time of no greater than 38ns.
 16. The scintillation crystal of claim 15, the scintillation crystalhas a decay time of no greater than 36 ns.
 17. The scintillation crystalof claim 16, wherein the second atomic concentration divided by thefirst atomic concentration is in a range of 0.4 to 2.5.
 18. Thescintillation crystal of claim 17, wherein the second dopant is Ca andhas a concentration in the scintillation crystal no greater than 160atomic ppm based on a total rare earth content in the scintillationcrystal.
 19. The scintillation crystal of claim 18, wherein the seconddopant has a concentration in the scintillation crystal of at least 11atomic ppm based on a total rare earth content in the scintillationcrystal.
 20. A radiation detection apparatus comprising: thescintillation crystal of claim 15; and a photosensor optically coupledto the scintillation crystal.